Hard copy printing may be accomplished in a variety of ways including offset techniques, rotogravure methods, photographic techniques, ink jet and thermographic printing. More recently, printing techniques have been developed which form an image as a pattern of electrical charges on a receptor or surface. Toner can then be applied to that surface for transfer to a paper sheet. Such techniques include xerography in which a photosensitive drum or plate is selectively discharged by exposure to a light image, as well as ion printing in which a stream of ionically charged gas molecules from an ion source is guided through a control grid to a receptor surface to create an ionic charge pattern on that surface. Some laser printers also write information on a photosensitive receptor surface by selectively discharging that surface using a scanning beam of coherent light. Each of these printing methods has certain advantages which suit specific markets. However, none of them are capable of producing a print with the quality of a photographic print at a very low cost per page and in a short time, i.e., seconds, using a simple, relatively inexpensive, variable data printer.
Xerographic printers probably come as close as any to satisfying all of these desirable criteria. The typical xerographic printer consists of an electrostatically chargable drum, a charging station for charging the drum, an exposing station at which an electrostatic image is applied to the drum surface, a toning station at which electroscopic powder or liquid is applied to the drum surface to develop or render visible the electrostatic image thereon, a transfer station where the toner image is transferred onto paper and a fusing station at which the toner particles are fused to the paper, while the drum is being readied for the next exposure at a cleaning station. Invariably, in electrostatic printers and copiers, the fusing station must be off-line because conventional electrostatic drums cannot tolerate heat. In other words, heat effects adversely the photoconductive material of the drum, causing, among other degradations, the loss of the charge image on the drum.
Also, because the different steps in the prior xerographic processes are carried out at different stations, these printers and copiers operate at relatively slow speeds, and, thus, have relatively low copying rates. Furthermore, they constitute fairly complex mechanical apparatus requiring elaborate optics and toner transport systems in the form of pumps and ancillary plumbing or toner powder dispensers, cleaning brushes, etc. Also, the quality of the copies produced by prior electrostatic and xerographic printing systems is still not as high as is desired, particularly in the case of those printers capable of printing in color.
Accordingly, it would be desirable to provide a printing system or press which can print consistently very high quality copies in black and white or in color on ordinary paper and do this economically and at high speed.